The present invention relates to an apparatus for determining the particle size distribution and for characterising the particle shapes of a particle mixture by electrooptical scanning with a metering device for classifying the particle mixture in the particle stream to be scanned and with an optoelectronic measurement section formed by a light source and an image collecting device, along which section there takes place a movement effected by gravitational force or additional forces of the particle stream between the light source and image collecting device and a digital recording of the projection areas in the form of image points covered by the projection areas of the particles in the image collecting device, and with a computing unit for converting the digital information into a size determination of the recorded particles.
An apparatus of the aforementioned type is described in the company prospectus "Partikelform und Gro.beta.enanalyse fur Labor und Produktion" (Particle shape and size analysis for laboratory & production) published by Me.beta.technik Schwarz GmbH, Dusseldorf. This apparatus includes a digital matrix camera that takes photographs, in the form of two-dimensional projection areas, of the particles moving through its image collecting area against the background of the light source, the accuracy of this recording being dependent on the exposure time and the depth of focus of the electrooptical image recording device that is used.
The so-called dynamic factor B, the quotient of the upper and lower measurement limits, serves as a measure of the measurement range of such a matrix camera. The larger for example the particles to be measured in the grain distribution to be investigated in the particle stream, the more image points there are covered by the particles in their passage through the image collection area of the matrix camera. Since within the scope of the computer-assisted evaluation all those particles are discounted that extend even only partially beyond the image collecting area, there is an increased probability that larger particles will be significantly under-represented in the investigation of corresponding grain distributions. In addition, larger and heavier particles have a higher fall velocity, with the result that, depending on the exposure time, there is the danger that the recorded projection area will be smudged. For the above reasons the dynamic factor is limited to B=35 to 40 with the widely employed CCD matrix cameras, so that in practice with a lens only those grain distributions having the necessary predictive accuracy can be investigated, in which the largest particle is at most 35-40 times as large as the smallest particle. If grain distributions having a width differing from the above or with another size range are to be investigated, then the image scale of the matrix camera must be matched to that of the grain distribution being investigated, which involves time-consuming and complicated manual activities such as changing lenses, adjusting the depth of focus, changing the camera position, and also adjusting the apparatus.
An apparatus of the aforedescribed type having an area camera is furthermore known from WO 97/14950 A1.
In an apparatus known from DE 41 19 240 C2 the image collecting device comprises a CCD line camera; since a line camera in each case records only the image of one chord length of the recorded particle, in order to obtain evaluable particle information the lines must be read in quick succession, in each case two-dimensional projection areas of the particles to be investigated being constructed from the individual one-dimensional image sequences. The dynamic factor can be set to B=100 in such line cameras, which means that a significantly broader grain distribution can be investigated than with a CCD matrix camera. However, the use of a line camera has the disadvantage that the projection areas of the measured particles are constructed from the line information under the assumption of a constant fall velocity of the relevant particles. This is not the case however, since during the scanning movement of the line camera the particles are undergoing an accelerated motion, and moreover particles of different size do not fall at the same velocity at the measurement site on account of air friction, but instead exhibit a velocity distribution. On account of these influencing factors, which are not negligible, such an apparatus does not operate with sufficient accuracy.
The object of the present invention is to improve an apparatus of the type mentioned at the beginning having regard to increasing the dynamic factor and the accuracy of the grain size determination.